Mark Pilgrim, author of "Greasemonkey Hacks: Tips & Tools for Remixing the Web with Firefox," gave a talk Tuesday afternoon at O'Reilly's ETech Emerging Technology Conference in San Diego to show off the promise (and peril) of the Firefox Web browser add-on, which is nearly at its one-year anniversary.

Greasemonkey is not a car mechanic, but a tool for manipulating and customizing the design of remote Web pages (without the knowledge of publishers) when sites are viewed through the popular open-source browser, Firefox. Pilgrim is an expert on the user JavaScript extension: He identified a major vulnerability in it last year that allowed attackers to view files on a user's hard drive.

After retelling the story of the security breach, Pilgrim highlighted some of his favorite Greasemonkey user scripts among the thousands out there, including his own "Butler," which was one of the original scripts that modified Google by stripping it of ads and adding links to rival search engines including Yahoo and Lycos.

Pilgrim's top Greasemonkey hacks? Book Burro is one, adding a script to Firefox that calls up comparison pricing whenever the user hunts for a book. For example, if the script detects the user is looking at a book sold at Amazon.com, it will automatically search rival bookstores, including BarnesandNoble.com, and list prices for the same book on the page.

Similarly, the Wikipedia proxy user script will add links to any Web page with relevant pointers to the community-based Wikipedia encyclopedia. The Greasemonkey hack called Omnifeedster will display the incoming links to any Web page on that page. The Omnifeeder hack is based on Feedster API.

Finally, the RIAA Radar is Pilgrim's professed favorite Greasemonkey adaptation, and what he calls a "passive activism" script. The RIAA Radar agent, whose code is less than one page long, will look up the sponsoring record label behind any album sold online and provide data on whether it supports the efforts of the Recording Industry Association of America (RIAA), which has notoriously filed lawsuits against people who have traded copyrighted songs online.

The script will detect when a user is looking to buy a record, and then automatically show an icon next to the record image to illustrate whether it is pro RIAA or an independent label. "It shows evil record companies," Pilgrim said to a packed audience at O'Reilly's conference this week.

"What's the future of Greasemonkey?," he asked, then answered that for end users, it's likely more of the same. And that is scripts to block ads online and fix broken Web sites. But sometimes, there will be break-out prototypes like Book Burro and CustomizeGoogle.